In django, named capturing groups are passed to your view as keyword arguments.
Unnamed capturing groups (just a parenthesis) are passed to your view as arguments.
The ?P is a named capturing group, as opposed to an unnamed capturing group.
http://docs.python.org/library/re.html
(?P<name>...)Similar to regular parentheses, but the substring
matched by the group is accessible within the rest of the regular
expression via the symbolic group name name. Group names must be valid
Python identifiers, and each group name must be defined only once
within a regular expression. A symbolic group is also a numbered
group, just as if the group were not named. So the group named id in
the example below can also be referenced as the numbered group 1.